To transfer eBooks that do NOT have any DRM protection, you do not need to download any software.
If you are transferring an eBook that is Adobe DRM protected, you need to download and install Adobe Digital Editions on your computer. This program is free to download. When you download Adobe Digital Editions (ADE), you’re getting the authorization needed to access books that are protected by Adobe Digital Rights Management (“DRM”). Adobe DRM prevents the unauthorized copying or redistribution of eBooks to protect the rights of authors and publishers. You can read Adobe DRM-protected files on up to 5 devices and 5 computers; check the Adobe website for more information.